Asymmetric photonic crystal waveguide element having symmetric mode fields

ABSTRACT

A slab photonic crystal waveguide that preserves the parity of guided modes. The waveguide includes a photonic crystal layer interposed between two dielectric layers. The photonic crystal layer includes a periodic arrangement of asymmetrically shaped dielectric regions within a surrounding dielectric material. The waveguide precludes conversion of the state of parity of an introduced input mode by maintaining a symmetric mode field distribution. A symmetric mode field distribution is attained through variations in the dielectric constants of the dielectric layers that compensate for asymmetric mode localization tendencies associated with the asymmetric periodically arranged dielectric regions within the photonic crystal layer.

FIELD OF INVENTION

This invention pertains to an element for waveguiding electromagneticradiation. More particularly, this invention relates to a slab waveguideelement that confines electromagnetic radiation laterally in a photoniccrystal layer and vertically through index guiding. Most particularly,this invention relates to a structurally asymmetric waveguide elementthat preserves the symmetry of guided modes.

BACKGROUND OF THE INVENTION

Attention recently has been focused on developing materials capable ofcontrolling the propagation of light in much the same way thatsemiconductors control the propagation of electrons. Over the pastdecade substantial progress has been made toward this goal and the newfield of photonic crystals has emerged. A photonic crystal functions asa semiconductor for light in the sense that it possesses a photonic bandgap that defines a range of electromagnetic frequencies that are unableto propagate in the crystal in one or more directions.

The ability of semiconductors to control the propagation of electronsoriginates from the periodic lattice arrangement of the atoms thatconstitute the semiconductor. The precise arrangement and spacing ofatoms ultimately dictates the electronic potential that underlies theband structure, bandgap and electronic states of a semiconductor.Periodicity is also used in photonic crystals to achieve a photonicbandgap and to control the density of photonic states at differentfrequencies. Instead of a periodic electronic potential originating fromperiodically arranged atoms, however, periodicity of the refractiveindex originating from a periodic arrangement of one dielectric mediumwithin another underlies the formation of a photonic bandgap. Sinceelectromagnetic radiation of interest in photonic applications has alonger wavelength than the electrons confined in a semiconductor, theperiodic spacing of the refractive index variation in a photonic crystalis larger than the periodic spacing of atoms in a semiconductor. When aphotonic bandgap forms, the wavelengths of electromagnetic radiationwithin the bandgap are those that are comparable to the periodic spacingin refractive index. Electromagnetic radiation having an energy withinthe photonic band gap and propagating in a direction defined by thephotonic band gap is blocked and unable to propagate in a photoniccrystal. When external light having an energy and direction ofpropagation within the photonic band gap is made incident to a photoniccrystal, it is unable to propagate through the crystal. Instead, it isperfectly reflected. Light with an energy or direction of propagationoutside of the photonic band gap, on the other hand, freely passesthrough the crystal (subject, of course to ordinary absorption andreflection processes). This feature makes photonic crystals essentiallyperfect reflectors of incident wavelengths that are within thewavelength range and range of propagation directions encompassed by thephotonic bandgap.

An example of a practical photonic crystal would be a material thatconsists of a flat dielectric slab that contains a periodic arrangementof holes extending in the thin direction and aligned along the lateraldimensions of the slab. Such a material may be viewed as a periodicarrangement of rods comprised of air and corresponds to a photoniccrystal in which air is the macroscopic dielectric medium and the slabis the surrounding medium. Another example of a photonic crystal wouldbe a periodic array of cylindrically shaped rods made of a dielectricmaterial supported by a substrate with the space between the rods beingfilled by air or a dielectric material other than the one from which therods are made. In this example, the rods correspond to the periodicallydistributed macroscopic dielectric medium and the material filling thespace between the rods corresponds to the surrounding matrix. Theprecise details of the periodic pattern of rods (or other shape) and therefractive index contrast between the periodic macroscopic dielectricmedium and its surroundings influences the properties of the photoniccrystal.

Photonic crystals can be formed from a wide variety of macroscopicdielectric media provided that an appropriate refractive index contrastwith a surrounding medium can be achieved. As an example, thecomposition of the holes or rods in the example above is not limited toair. Other materials that present a sufficiently large refractive indexcontrast with the surrounding flat dielectric slab may be used to formthe rods. A periodic lattice of air holes, for example, may be drilledin a flat dielectric slab and subsequently filled with another materialto form a photonic crystal. The rod material may have a higher or lowerrefractive index than the slab material. As another example, a periodicarray of rods comprised of a macroscopic dielectric medium such assilicon in air as the surrounding medium represents a photonic crystal.

Important material design considerations include the size, spacing andarrangement of macroscopic dielectric media within a volume ofsurrounding material as well as the refractive indices of the dielectricand surrounding materials. The periodicity of the macroscopic dielectricmedia can extend in one, two or three dimensions. These considerationsinfluence the magnitude of the photonic band gap, the frequency range oflight or other electromagnetic energy (e.g. infrared, microwave etc.)that falls within the photonic band gap and whether the photonic bandgap is full (in which case the photonic band gap effect is manifestedregardless of the direction of propagation of the incident light) orpartial (in which case the photonic band gap effect is manifested forsome, but not all, directions of propagation). Other practicalconsiderations are also relevant such as manufacturability, cost,ability to fabricate a periodic array of rods etc. Effects analogous todoping or defects in semiconductors may also be realized in photoniccrystals. An inherent consequence of dopants or defects insemiconductors is a disruption or interruption of the periodicity of thelattice of atoms that constitute the semiconductor. The electronicstates associated with dopants or defects are a direct consequence ofthe local disturbance in periodicity imparted to the semiconductorlattice. Photonic crystals can similarly be perturbed in ways analogousto introducing dopants and defects in semiconductors. Defects can beused to spatially confine light within a photonic crystal. A pointdefect can be used to localize electromagnetic radiation having awavelength within the photonic bandgap. This occurs because thelocalized electromagnetic radiation is unable to escape from the defectdue to its inability to propagate into or through the surroundingphotonic crystal by virtue of the fact that the localized wavelength iswithin the photonic bandgap. Linear and planar defects can similarly beused to confine electromagnetic radiation in one or two dimensionswithin a photonic crystal.

The periodicity of a photonic crystal is a consequence of a regular andordered arrangement of macroscopic dielectric media within a surroundingmedium. Effects that interrupt the arrangement of macroscopic dielectricmedia can be used to break the periodicity to create localized orextended defect photonic states within the photonic band gap. Defectscan be formed in rod array photonic crystals, for example, by perturbingone or more of the rods with respect to other rods in an array. Possibleways of perturbing rods in a surrounding dielectric slab, for example,include varying the size, position, optical constants, chemicalcomposition of one or more rods or forming rods from two or morematerials. Perturbation of a single rod provides a point defect that canbe used to localize light. Perturbation of a row of rods provides alinear defect that acts to confine light in a channel. Such defects canbe used to efficiently transfer light through the crystal withoutlosses.

As the field of photonic crystals develops, the need for new photonicband gap materials is increasing. An important potential area ofapplication for photonic crystals is waveguiding. In an ideal waveguide,a propagating beam of electromagnetic radiation is totally confined to adirection dictated by the waveguide. A three-dimensional photoniccrystal offers an approach for achieving total confinement and losslesspropagation of light. Waveguiding can be achieved in a three-dimensionalphotonic crystal by including a linear defect in the interior of thecrystal. Light localized in the defect is confined to the defect if thewavelength is within the photonic bandgap of the surrounding photoniccrystal. This occurs because the light is unable exit the defect andenter the surrounding photonic crystal. Three-dimensional photoniccrystals are desirable for waveguiding applications because the photonicbandgap is complete in the sense that the effects of the photonicbandgap effects are manifest regardless of the direction of propagationof the light having a wavelength within the gap. Full three-dimensionalconfinement by the photonic bandgap over a full range of propagationdirections is achievable and transmission losses are avoided.Three-dimensional photonic crystals are thus a highly valuable targetfor compact integrated optics systems which necessarily require sharpbends to minimize system size. In the absence of a three-dimensionalphotonic bandgap, prohibitive losses would occur at bends as thepropagating light travels in a direction that fall outside of thebandgap.

A current outstanding problem, however, is the practical difficulty ofachieving a full three-dimensional photonic crystal. A complete photonicbandgap requires the construction of a three-dimensional photoniccrystal. The exacting requirements for periodically arrangingmacroscopic dielectric objects having a size on the order of thewavelength of propagating light has proven to be both challenging andcostly.

A need exists for photonic crystals whose performance approaches thatexpected for three-dimensional photonic crystals and whose manufactureis less demanding. One solution that has been proposed is the slabphotonic crystal. A slab photonic crystal includes a photonic crystallayer having a finite thickness and including a periodic array in twodimensions of one dielectric material within a surrounding dielectricmaterial having a different composition. The layered structure of theslab photonic crystal makes its construction amenable to widelyavailable layered deposition and processing techniques.

Periodicity in a slab photonic crystal occurs in the two lateral (in theplane of slab) dimensions, but is absent in the direction normal to theslab (i.e. thickness direction). Since a photonic crystal layer of thistype is not periodic in three dimensions, it lacks a complete bandgap.This means that the photonic bandgap is operable with respect toincluded wavelengths only over a particular range of propagationdirections. Wavelengths that are nominally within the bandgap areexcluded from the bandgap for directions of propagation that falloutside of those encompassed by the bandgap. In this sense, theconfinement of light by a defect in a photonic crystal layer isincomplete since the confinement is effective only for a limited rangeof propagation directions.

In a slab photonic crystal, the confinement of light is made complete byinterposing a photonic crystal layer between two lower index dielectriccladding layers. The purpose of the cladding layers is to provideconventional index confinement of light that falls outside of thephotonic bandgap of the photonic crystal layer. In this way, light canbe maintained within the combination of layers without incurringsubstantial losses. The slab photonic crystal layer provides confinementfor lateral propagation directions (directions of periodicity of thephotonic crystal layer), while the cladding layer provides confinementin the slab normal direction.

A practical problem commonly encountered in the fabrication of slabphotonic crystals is an asymmetry in the shape of the periodicallyarranged dielectric medium in the direction normal to the slab. In atypical example, the photonic crystal layer of a slab photonic crystalincludes a periodic array of macroscopic rods comprised of a firstdielectric material within a surrounding matrix of a second dielectricmaterial. In the planar processing of such a photonic crystal layer,processing occurs by etching holes in a solid piece of the second(surrounding) dielectric material and subsequently filling these holeswith the first dielectric material. Due to the nature of the etchingprocess, the holes that form are not precisely cylindrical, but ratherslightly conical or tapered so that the top of the hole is wider thanthe bottom part of the hole. The tapering is a consequence of the factthat the upper part of the hole is more readily accessed by the etchant,while the lower part of the hole is more difficult to access. Etchingtherefore occurs most efficiently at the top surface and becomesprogressively less efficient due to inhibited access of the etchant awayfrom the surface toward the bottom of the hole.

A consequence of the tapering is that the cross-sectional shape and/orarea of the periodically arranged dielectric regions is non-uniform inthe slab normal direction. This non-uniformity has a deleterious effecton waveguiding because it represents a destruction of mirror symmetrywith respect to the mid-plane of the slab. This loss of symmetry leadsto a mixing of guided modes of different parity and as a result, singlemode waveguiding is precluded. Instead, mode coupling and multimodetransmission occur with an accompanying increase in losses due toreflection. In order to improve the transmission efficiency of slabphotonic crystals, it is desirable to devise a system that preservesmirror symmetry so that single mode operation can be achieved.

SUMMARY OF THE INVENTION

The instant invention provides a structurally asymmetric slab photoniccrystal that preserves mode symmetry to permit single mode operation andefficient transmission of guided modes. The instant slab photoniccrystal waveguide includes a photonic crystal layer having a periodicarrangement of discrete regions of a first dielectric material within asurrounding second dielectric material, where, in a preferredembodiment, the discrete regions of the first dielectric material areequivalent to each other with each having a tapered or otherwisenon-uniform cross-section in the slab-normal direction. The photoniccrystal layer is interposed between two dielectric cladding layers thatdiffer in dielectric constant. The cladding layers provide confinementof guided modes in the slab-normal direction and the difference indielectric constants of the cladding layers are selected in such a waythat the cladding layers compensate for the asymmetry in the shape ofthe periodically arranged dielectric regions to provide for symmetricconfinement of guided modes.

In a preferred embodiment, the photonic crystal layer includes aperiodic arrangement in two dimensions of tapered rods having asubstantially circular cross-section in planes aligned with the photoniccrystal layer where the diameter of the circle varies in the slab-normaldirection. The tapered rods are arranged within a surrounding dielectricmaterial having a lower dielectric constant than the tapered rods. Inthis embodiment, the cladding layer adjacent to the narrower end of thetapered rod has a higher dielectric constant than the cladding layeradjacent to the wider end of the tapered rod.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Schematic depiction of a slab photonic crystal waveguide.

FIG. 2A. Side view of a photonic crystal layer.

FIG. 2B. Top view of a photonic crystal layer.

FIG. 2C. Side view of a slab photonic crystal waveguide.

FIG. 2D. Mode field distribution within a slab photonic crystalwaveguide in side view.

FIG. 3A. Side view of an asymmetric slab photonic crystal waveguide.

FIG. 3B. Mode field distribution within an asymmetric slab photoniccrystal waveguide in side view.

FIG. 4A. Side view of a slab photonic crystal including upper and lowerdielectric layers having different dielectric constants.

FIG. 4B. Side view of the mode field distribution within a slab photoniccrystal that includes upper and lower dielectric layers having differentdielectric constants.

FIG. 5A. Side view of an asymmetric slab photonic crystal includingupper and lower dielectric layers having different dielectric constants.

FIG. 5B. Top view of the photonic crystal layer of the embodiment shownin FIG. 5A.

FIG. 5C. Bottom view of the photonic crystal layer of the embodimentshown in FIG. 5A.

FIG. 5D. Side view of the mode field distribution within an asymmetricslab photonic crystal that includes upper and lower dielectric layershaving different dielectric constants.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

A schematic depiction of a slab photonic crystal waveguide in side viewis provided in FIG. 1. The slab waveguide includes upper dielectriclayer 100, lower dielectric layer 200 and photonic crystal layer 300.The z-direction indicated is a direction normal to the photonic crystallayer 300 and may be referred to herein as the slab-normal, normal,thickness or vertical direction. The lateral directions are included inthe xy-plane (not shown) which is normal to the plane of the page ofFIG. 1. The x and y directions may be referred to herein as the in-slab,horizontal or lateral directions. Confinement of guided modes in thein-slab directions is provided by the photonic bandgap and confinementof guided modes in the slab-normal direction is provided by thedielectric layers 100 and 200. The dielectric layers 100 and 200 mayalso be referred to herein as cladding layers. Confinement in theslab-normal direction requires that the dielectric constants of thecladding layers be less than the dielectric constant of the photoniccrystal layer.

In an idealized slab photonic crystal waveguide, discrete regions of afirst dielectric material are periodically arranged in two dimensionswithin a surrounding matrix of a second dielectric material where theperiodically arranged discrete regions are identical in shape and size.The discrete regions are arranged periodically in the two in-slabdirections and extend continuously in the slab-normal direction betweenthe upper and lower cladding layers. In order to preserve mode symmetry,the upper and lower cladding layers have the same dielectric constantand the periodically arranged discrete regions have a shape that issymmetric with respect to the mid-plane of the photonic crystal layer.(The mid-plane of the photonic crystal layer is the plane that passesthrough the photonic crystal layer and is located halfway between theupper and lower surfaces of the photonic crystal layer. In the slabphotonic crystal waveguide shown in FIG. 1, for example, the mid-planeof the photonic crystal layer is a horizontal plane that bisects thephotonic crystal layer.)

In FIG. 2A is depicted in side view an enlargement of a representativeembodiment of a photonic crystal layer for an idealized slab photoniccrystal waveguide. In this embodiment, the photonic crystal layer 310includes periodically arranged dielectric regions 305 surrounded bydielectric material 315. A top view of the idealized photonic crystallayer is shown in FIG. 2B. The top view shows the periodic arrangementof dielectric regions 305 within the surrounding dielectric material315. The top view represents the perspective of the xy-planes (in slabplanes; planes normal to the slab normal direction). The bottom view ofthe dielectric regions 305 matches the top view shown in FIG. 2B. Theviews shown in FIGS. 2A and 2B indicate that in this embodiment, thedielectric regions 305 are rods having a cylindrical shape that issymmetric with respect to the mid-plane of the photonic crystal layer310.

FIG. 2C shows a side view of a slab photonic crystal waveguide 320 thatincludes the photonic crystal layer 310 depicted in FIGS. 2A and 2B. Inaddition to photonic crystal layer 310 which includes symmetricrod-shaped dielectric regions 305 periodically distributed withinsurrounding dielectric material 315, the slab waveguide further includesan upper dielectric layer 325 and a lower dielectric layer 335. Upperand lower dielectric layers 325 and 335 may also be referred to ascladding layers. Mid-plane 345 is also shown and corresponds to a planethat horizontally bisects photonic crystal layer 310. Mid-plane 345 islocated halfway between the interface of the photonic crystal layer 310with the upper dielectric layer 325 and the interface of the photoniccrystal layer 310 with the lower dielectric layer 335. Mid-plane 345 isparallel to the upper dielectric layer 325 and lower dielectric layer335. The symmetry of the rod-shaped dielectric regions 305 with respectto mid-plane 345 is evident in FIG. 2C. The symmetry can be viewed interms of a mirror symmetry in which a reflection of the portion of therod-shaped regions 305 above mid-plane 345 through mid-plane 345 mapsthat upper portion into the lower portion (the portion below mid-plane345) of the rod-shaped regions 305. Symmetry with respect to mid-plane345 can also be viewed in terms of a superimposability of the upperportion of rod-shaped regions 305 on the lower portion of rod-shapedregions 305. Periodically arranged dielectric regions that possesssuperimposability of the portions above and below the mid-plane aresymmetric, while periodically arranged dielectric regions that lacksuperimposability with respect to the mid-plane are asymmetric.

Mode localization within the slab photonic crystal waveguide 320 dependson the relative values of the dielectric constants of the differentregions of the waveguide. In principle, different materials can be usedfor the symmetric rod-shaped regions 305, surrounding dielectricmaterial 315, upper dielectric layer 325 and lower dielectric layer 335.As a result, the relative dielectric constants of the different regionscan be varied in a number of ways to control the mode field distributionwithin the waveguide. In the instant invention, it is preferred toconfine guided modes within the photonic crystal layer 310 and toachieve lateral (horizontal) confinement through the photonic bandgap ofthe photonic crystal layer 310 and slab-normal (vertical) confinementthrough the upper and lower dielectric layers 325 and 335.

Within the photonic crystal layer 310, the dielectric constant of thesymmetric rod-shaped regions 305 can be greater than or less than thedielectric constant of the surrounding dielectric material 315. Theguided mode will preferentially localize in whichever region has thehigher dielectric constant. By insuring that the upper and lowerdielectric layers 325 and 335 have a lower dielectric constant than theregions (symmetric rod-shaped regions 305 vs. surrounding dielectricregion 315) within the photonic crystal layer in which the mode islocalized, confinement in the slab-normal (vertical) direction isachieved.

By requiring the cladding layers 325 and 335 to have the same dielectricconstant, the guided mode supported within the photonic crystal layer310 has a mode field distribution that is symmetric with respect to aninternal plane of mirror symmetry that coincides with the horizontalmid-plane 345 of the waveguide structure. Depending on which of thesymmetric rod-shaped regions 305 or surrounding dielectric region 315has the higher dielectric constant, the mode field distribution of thesupported mode can have even (dielectric constant of regions315>dielectric constant of regions 305) or odd (dielectric constant ofregions 305>dielectric constant of regions 315) parity with respect tothe internal plane of symmetry.

FIG. 2D shows the mode field distribution in side view for an embodiment420 of the slab photonic crystal waveguide depicted in FIG. 2C. In thisembodiment, the dielectric constants of the upper dielectric layer 425and lower dielectric layer 435 are equal and are designated ε_(c). Theupper and lower dielectric layers 425 and 435 may also be referred to ascladding layers. The dielectric constant of the surrounding dielectricregions 415 is also chosen to be ε_(c), while the dielectric constant ofthe symmetric rod-shaped dielectric regions 405 is chosen to be ε_(r),where ε_(r)>ε_(c). The mode field distribution 455 shows the spatialdistribution of the electric field intensity of a guided mode supportedby the slab photonic crystal. Since ε_(r)>ε_(c), the mode field isconcentrated in the rod-shaped regions 405. With the relative values ofthe dielectric constant chosen for this embodiment, the mode fielddistribution possesses odd parity with respect to the horizontalmid-plane of symmetry 445 that bisects photonic crystal layer 410. Themode field distribution 455 as depicted in FIG. 2D represents theprimary area of localization of a mode supported by the slab photoniccrystal. It is understood by persons of skill in the art that the modefield includes evanescent contributions that extend beyond theboundaries of the dielectric regions 405.

In the practical fabrication of slab photonic crystal waveguides, it isoften difficult to maintain a symmetric shape for the periodicallydistributed dielectric regions included within the photonic crystallayer. In a typical fabrication process, the processing begins withdepositing or otherwise forming a layer of the surrounding dielectricmaterial of the photonic crystal layer onto the lower dielectric layer.In order to introduce the periodically arranged dielectric regions intothe photonic crystal layer, it is necessary to remove portions of thelayer of the surrounding dielectric material to create the spaces inwhich to add the periodically arranged dielectric regions. The goal inremoving portions of the layer of the surrounding dielectric material isto create holes or cavities into which a second material can be added ordeposited within the surrounding dielectric material to produce aphotonic crystal layer. The holes must necessarily be periodicallypositioned and preferably are identical in size and shape. In order toachieve symmetrically shaped periodically arranged dielectric regions,it is necessary that the formation of holes or cavities within the layerof the surrounding dielectric material be uniform in the depthdirection.

From a processing standpoint, a preferred method in many processenvironments for creating the holes is etching. By masking the topsurface of the layer of surrounding dielectric material, a periodicallyarranged distribution of unmasked regions can be defined at the surfaceand these unmasked regions can be exposed to a suitable etchant tocreate holes or cavities within the layer of surrounding dielectricmaterial. The masking can also be used to define the cross-sectionalshape (e.g. circular, square, groove) of the holes or cavities that willbe filled with a different dielectric material to provide theperiodically arranged dielectric regions of the photonic crystal layer.In order to achieve symmetrically shaped holes (as would be necessary toform a slab photonic crystal waveguide such as that depicted in FIG.2D), it is necessary that etching occur uniformly in the depth directionso that the cross-sectional shape and area remains constant in the depthdirection. When these conditions are met, the individual dielectricregions in the periodic arrangement are symmetric with respect to ahorizontal plane of symmetry that bisects the photonic crystal layer.

In practice, a uniform etch in the depth direction is difficult toachieve because of the differential in the time of exposure that occursas a function of depth. The etching initiates at the top surface of thelayer of surrounding dielectric material and continues into the interiorof that layer toward the interface with the lower dielectric material.As a result, the time of contact of the etchant is greatest at the topsurface of the layer of surrounding dielectric material, progressivelydecreases in the depth direction away from the top surface, and is at aminimum at the interface with the lower dielectric material. Theresulting differential in the time of exposure to the etchant in thedepth direction tends to produce a tapered or otherwise asymmetric ornon-uniform cavity cross-section as a function of depth in the layer ofsurrounding dielectric material. The cross-sectional shape and/ordimensions may vary in the depth direction and destroy the symmetry withrespect to the horizontal plane described hereinabove.

In a typical situation, the cavities in which the periodically arrangeddielectric regions are introduced are tapered and have a largercross-section at the top surface of the layer of surrounding dielectricmaterial (the surface that ultimately forms the interface with the upperdielectric layer) and a smaller cross-section at the bottom surface ofthe layer of surrounding dielectric material (the surface that forms theinterface with the lower dielectric layer). Such cavities (and theperiodically arranged dielectric materials that fill them) lack symmetrywith respect to the horizontal mid-plane of the photonic crystal layerand may be referred to herein as asymmetric cavities or asymmetricregions of periodically arranged dielectric materials or the like. Aslab photonic crystal that contains asymmetrically shaped periodicallyarranged dielectric materials may be referred to herein as an asymmetricslab photonic crystal.

A representative example of an asymmetric slab photonic crystal is shownin side view in FIG. 3A. The asymmetric slab photonic crystal 520includes upper dielectric layer 525, lower dielectric layer 535, andphotonic crystal layer 510 that includes surrounding dielectric material515 and periodically arranged dielectric regions 505. The upper andlower dielectric layers 525 and 535 may also be referred to as claddinglayers. The periodically arranged regions 505 lack symmetry with respectto the mid-plane of photonic crystal layer 510 and are accordinglyreferred to as asymmetric. The dielectric constants of the upperdielectric layer 525, lower dielectric layer 535 and surroundingdielectric material 515 are selected to be equal and are designatedε_(c) in FIG. 3A. The dielectric constant of the periodically arrangedregions 505 is designated ε_(r), where ε_(r)>ε_(c) in the embodimentshown in FIG. 3A. The tapered shape of the periodically arrangeddielectric regions 505 is evident and exemplifies the shapes resultingfrom typical etching processes.

The tapered, asymmetric shape of the periodic dielectric regions 505influences the mode field distribution of guided modes supported byphotonic crystal layer 510. Since the periodic dielectric regions 505have a higher dielectric constant than the surrounding dielectricregions 515, the mode is preferentially localized in the periodicregions 505 and since the periodic regions 505 are asymmetric, the modefield distribution is asymmetric. The mode field preferentiallylocalizes in the wider, upper portion of the periodic regions 505 and asa result, the mode field distribution is asymmetric with respect to ahorizontal mid-plane of the photonic crystal layer 510. A schematicdepiction of the mode field in side view for the embodiment shown inFIG. 3A is presented in FIG. 3B. The mode field 555 is preferentiallylocalized in the upper, wider portion of periodic regions 505 and isasymmetric with respect to the horizontal mid-plane 545. The mode fielddistribution 555 as depicted in FIG. 3B represents the primary area oflocalization of a mode supported by the slab photonic crystal. It isunderstood by persons of skill in the art that the mode field includesadditional evanescent contributions.

A deleterious consequence of mode field asymmetry is that the parity ofguided modes is no longer purely even or purely odd as in the case ofsymmetric mode fields. Instead, both odd and even parity modes can besupported in the photonic crystal layer and a mixing or coupling ofmodes of different parity is possible. If a mode having a state ofdefinite parity is introduced into an asymmetric slab photonic crystal,the crystal permits a conversion of the parity state into other paritystates and the purity or definiteness of parity is lost. An even paritymode, for example, that is introduced into an asymmetric slab photoniccrystal can be transformed into an odd parity mode or a linearcombination of even and odd parity modes. Instead of single modetransport through the crystal, multimode transport occurs as power fromthe entering mode is coupled into multiple modes that become sustainablein the crystal due to its asymmetric condition. As a result, the powerof a pure mode (a mode having a pure or definite state of parity) isredistributed to multiple modes and the transmitted power in the initialmode state is significantly reduced. In effect, the presence ofasymmetry operates as a source of power loss with respect to aparticular state of mode parity through mode coupling to other paritystates.

The instant invention provides a slab photonic crystal that preservesmode parity in the face of asymmetric periodically arranged dielectricregions in the photonic crystal layer. The instant slab photonic crystalprovides a compensation mechanism that acts to offset the preferentiallocalization of mode field that occurs in the wider, highcross-sectional area portion of tapered or otherwise asymmetricperiodically arranged dielectric regions. Compensation is effectedthrough an independent mechanism of varying mode field localization thatis produced by including upper and lower dielectric layers in anasymmetric slab photonic crystal that have different dielectricconstants. By tailoring the relative values of the dielectric constantsof the upper and lower dielectric layers, an asymmetry in dielectricconstant adjacent to the photonic crystal layer is created that offsetsthe asymmetry in mode field distribution resulting from tapered orotherwise non-uniform periodic dielectric regions. Through thiscompensation mechanism, a symmetric mode field distribution isachievable in a slab photonic crystal that includes periodicallyarranged dielectric regions that are asymmetric.

An illustration of the effect of a differential in the dielectricconstants of the upper and lower dielectric layers of a slab photoniccrystal is shown in side view in FIG. 4A. The photonic crystal 620includes upper dielectric layer 625, lower dielectric layer 635, andphotonic crystal layer 610 that includes surrounding dielectric material615 and periodically arranged dielectric regions 605. The upper andlower dielectric layers 625 and 635 may also be referred to as claddinglayers. The dielectric constants of the upper dielectric layer 625 andsurrounding dielectric material 615 are selected to be equal and aredesignated ε_(c1). The dielectric constant of the periodically arrangedregions 605 is designated ε_(r). The dielectric constant of the lowerdielectric layer 635 is designated ε_(c2), where the relative values ofthe dielectric constants are ε_(r)>ε_(c2)>ε_(c1). This relative orderingmaintains vertical confinement and further permits an additional degreeof freedom in controlling mode field distribution within the photoniccrystal layer 610.

In the embodiment of FIG. 4A, the periodic regions 605 are symmetric,but the different dielectric constants of the upper and lower dielectriclayers 625 and 635 lead to a deviation of the mode field distributionrelative to the symmetric distribution shown in FIG. 2D hereinabove. Themode field distribution 655 of the slab photonic crystal shown in FIG.4A is depicted in side view in FIG. 4B, where it is shown that the modefield distribution 655 becomes asymmetric and biased in the direction ofthe lower dielectric layer. The mode field distribution 655 as depictedin FIG. 4B represents the primary area of localization of a modesupported by the slab photonic crystal. It is understood by persons ofskill in the art that the mode field includes additional evanescentcontributions.

The higher dielectric constant of the lower dielectric layer relative tothe upper dielectric layer induces an asymmetry in the mode fielddistribution 655 and causes the mode field preferentially localizes inthe lower portion of the periodic regions 605. The greater thedifferential in the dielectric constants of the upper and lowerdielectric layers is, the greater is preference of the mode field tolocalize in the vicinity of the dielectric layer having the higherdielectric constant. The degree of asymmetry of the mode fielddistribution may thus be continuously varied and controlled by adjustingthe differential in the dielectric constants of the upper and lowerdielectric layers of a slab photonic crystal.

The mode field distribution 655 depicted in FIG. 4B lacks symmetry withrespect to an internal horizontal plane of the photonic crystal layer610. As a result, mode purity is not preserved in the slab photoniccrystal 620. Instead, modes of even and odd parity are mixed or coupledby the slab photonic crystal 620 and a mode having a definite state ofparity suffers a significant power loss upon introduction into the slabphotonic crystal 620 as the mode power is redistributed into other modeshaving different states of parity or linear combinations thereof.

The examples shown in FIGS. 4A and 4B demonstrate that variations in therelative dielectric constants of the upper and lower dielectric layerscan be used as an independent mechanism for influencing the mode fieldlocalization. In the instant invention, asymmetric slab photoniccrystals are provided in which mode field asymmetry produced byasymmetric periodically arranged dielectric regions within the photoniccrystal layer are offset by a compensating mode field asymmetry producedthrough a variation in the relative values of the dielectric constantsof the upper and lower dielectric layers.

As shown in FIG. 3B hereinabove, the presence of asymmetrically shapedperiodically arranged dielectric regions leads to a preferentiallocalization of the mode field in the wider portion of taperedperiodically arranged dielectric regions. The slab photonic crystalshown in FIG. 4B indicates that mode field localization can beindependently biased in the direction of the dielectric layer having thehigher dielectric constant. FIG. 5A shows an embodiment in side view ofthe instant invention that utilizes a differential in the dielectricconstants of the upper and lower dielectric layers to offset orcompensate an asymmetry in mode field distribution that results fromasymmetrically shaped periodically arranged dielectric regions in thephotonic crystal layer of a slab photonic crystal. The slab photoniccrystal 720 includes upper dielectric layer 725, lower dielectric layer735, and photonic crystal layer 710 that includes surrounding dielectricmaterial 715 and periodically arranged dielectric regions 705. The upperand lower dielectric layers 725 and 735 may also be referred to ascladding layers. The dielectric constants of the upper dielectric layer725 and surrounding dielectric material 715 are selected to be equal andare designated ε_(c1). The dielectric constant of the periodicallyarranged regions 705 is designated ε_(r). The dielectric constant of thelower dielectric layer 735 is designated ε_(c2), where the relativevalues of the dielectric constants are ε_(r)>ε_(c2)>ε_(c1). Thisrelative ordering maintains vertical confinement and further permits anadditional degree of freedom in controlling mode field distributionwithin the photonic crystal layer 710.

The top and bottom views of the photonic crystal layer 710 are shown inFIGS. 5B and 5C, respectively. Each of the views includes photoniccrystal layer 710 along with the periodically arranged dielectricregions 705 and surrounding dielectric regions 715. The cross-section ofperiodically arranged regions 705 is larger in the top view shown inFIG. 5B than in the bottom view shown in FIG. 5C due to the asymmetric,tapered shape of periodically arranged dielectric regions 705.

The mode field distribution of the embodiment depicted in FIGS. 5A, 5Band 5C is shown in side view in FIG. 5D. FIG. 5D shows slab photoniccrystal 720, upper dielectric layer 725, lower dielectric layer 735,photonic crystal layer 710, periodically arranged regions 705, andsurrounding dielectric regions 715. FIG. 5D further shows mode fielddistribution 755 and horizontal plane 745. The mode field distribution755 as depicted in FIG. 5D represents the primary area of localizationof a mode supported by the slab photonic crystal. It is understood bypersons of skill in the art that the mode field includes additionalevanescent contributions.

The mode field 755 is symmetrically distributed about horizontal plane745, with equal localization of the mode field 755 above and below theplane 745. The plane of symmetry 745 is contained within the photoniccrystal layer 710, but is displaced from the mid-plane location. Asshown in FIG. 5D, the plane of symmetry 745 in this embodiment islocated closer to lower dielectric layer 735 than to upper dielectriclayer 725. The displacement of the plane of symmetry 745 in thedirection of the lower dielectric layer 735 is due to the higherdielectric constant of the lower dielectric layer 735 relative to theupper dielectric layer 725.

Mode field 755 is symmetrically localized about symmetry plane 745.Symmetry of the mode field distribution 755 is a consequence of abalancing of effects related to the asymmetric shape of the periodicallyarranged dielectric regions 705 and the differential in dielectricconstant between the upper dielectric layer 725 and lower dielectriclayer 735. The tendency of the mode field to localize in the widerportion of the tapered periodically arranged dielectric regions of thephotonic crystal layer is offset by the tendency of the mode field tolocalize in the vicinity of the higher dielectric constant lowerdielectric layer. By adjusting the difference in the dielectricconstants of the upper and lower dielectric layers of a slab photoniccrystal appropriately relative to the asymmetry in the shape of theperiodically arranged dielectric regions, the slab photonic crystal canbe made to support a symmetric mode field. Preservation of the symmetryof the mode field distribution about a plane of symmetry is achieved andprevents the mixing or coupling of modes. As a result, a slab photoniccrystal having asymmetrically shaped periodically arranged can maintainthe purity of the state of parity of a guided mode utilizing the instantinvention. A mode having a definite state of parity (e.g. an even modeor an odd mode) retains its parity as it is guided through theembodiment depicted in FIGS. 5A–5D and does not suffer power losses dueto dissipation of energy into other modes.

The embodiment depicted in FIGS. 5A–5D is illustrative of the generalprinciples of the instant invention. The instant invention provides slabphotonic crystals that include asymmetrically shaped periodicallyarranged dielectric regions while providing for symmetric mode fielddistributions due to a compensating mode localization effect provided bya difference in the dielectric constants of the upper and lowerdielectric layers. Symmetry of the mode field distribution is withrespect to an internal plane of the photonic crystal layer, where theinternal plane may or may not coincide with the mid-plane of thephotonic crystal layer.

A preferred embodiment of the instant invention is one in which thesurrounding dielectric material and the periodically arranged dielectricregions of the photonic crystal layer are comprised of solid dielectricmaterials. In another preferred embodiment, the surrounding dielectricmaterial is a solid dielectric material and the periodically arrangedregions are comprised of a gas (e.g. air), so that the instant inventionincludes embodiments in which the photonic crystal layer includes aperiodic arrangement of holes (dielectric regions filled with air oranother gas) within a surrounding solid dielectric material. Additionalembodiments of the instant invention include embodiments in which theperiodically arranged dielectric regions and/or the surroundingdielectric material are liquid dielectric materials.

As indicated hereinabove, depending on the relative values of thedielectric constants of the periodically arranged dielectric regions andthe surrounding dielectric material of the photonic crystal layer, theguided mode supported by the instant slab photonic crystal waveguide maybe preferentially localized in either the periodically arrangeddielectric regions or the surrounding dielectric material. In anembodiment that includes a periodic arrangement of holes within asurrounding solid dielectric material, the mode field willpreferentially localize in the surrounding dielectric material. In suchan embodiment, if the holes are tapered downward (i.e. the holes have alarger cross-section at the interface with the upper dielectric layerthan at the interface with the lower dielectric layer), the regions ofsurrounding dielectric material necessarily have a greatercross-sectional area at the interface with the lower dielectric layerthan at the interface with the upper dielectric layer and as a result,the supported mode will preferentially localize in the portion of thesurrounding dielectric material in closer proximity to the lowerdielectric layer. In this embodiment, the compensation effect describedhereinabove is thus achieved by increasing the dielectric constant ofthe upper dielectric layer relative to the dielectric constant of thelower dielectric layer.

Although the embodiments described hereinabove have consideredperiodically arranged dielectric regions having a circularcross-section, the principles of the instant invention extend generallyto asymmetrically-shaped periodically arranged dielectric regions of anycross-sectional shape, including triangular, square, rectangular,elliptical, oval, and polygonal. Asymmetric periodically arrangeddielectric regions within the scope of the instant invention includeregions having a cross section that varies in size or shape in the depth(thickness) direction of the photonic crystal layer. As described in theembodiment depicted in FIGS. 5A–5D, an example of an asymmetricdielectric region is a region that has a circularly shaped cross sectionthat varies in size (diameter) in the thickness direction of thephotonic crystal layer between the upper and lower dielectric layers.Asymmetric dielectric regions may be similarly formed from other crosssectional shapes. In other embodiments, the periodically arrangeddielectric regions are asymmetric due to a change in cross sectionalshape in the depth direction. The cross sectional shape of theperiodically arranged dielectric region may be different at theinterface of the photonic crystal layer with the lower dielectric layerthan at the interface of the photonic crystal layer with the upperdielectric layer with a transformation from one cross sectional shape tothe other cross sectional shape occurring within the photonic crystallayer.

The upper and lower dielectric layers of the instant slab photoniccrystals can be homogeneous materials having a desired dielectricconstant or a heterogeneous or composite material that has an average oreffective dielectric constant that provides confinement and mode fieldlocalization effects in accordance with the instant invention. The upperand lower dielectric layers are comprised of a dielectric material. Thedielectric material can be a solid (e.g. silica, glass, titania,silicon, etc.), a liquid (e.g. water or other polar liquid, ahydrocarbon or other non-polar liquid etc.) or a gas (e.g. air,nitrogen, argon, etc.).

Further embodiments of the instant invention include those in which themode field is localized preferentially in the surrounding dielectricmaterial rather than preferentially in the periodically arrangeddielectric regions as described in the selected illustrative exampleshereinabove. These embodiments are realized by including a surroundingdielectric material that has a higher dielectric constant than theperiodically arranged dielectric regions of the photonic crystal layer,while maintaining confinement by the upper and lower dielectric layers.

Among the embodiments considered hereinabove are embodiments in whichthe parity of an odd parity mode supported by the instant slab photoniccrystal waveguides is defined with respect to a single nodal plane.Further embodiments of the instant invention include slab photoniccrystal waveguides that support symmetric even or odd modes, where modeparity is defined with respect to two or more nodal planes. In thesefurther embodiments, the plane of symmetry of a symmetric even or oddparity mode field distribution may or may not coincide with a nodalplane.

The instant invention further includes a system for transmittingelectromagnetic radiation that includes a source of electromagneticradiation coupled to the instant slab photonic crystal waveguide wherethe source introduces electromagnetic radiation into the slab photoniccrystal waveguide and the introduced electromagnetic radiation is guidedthrough the waveguide by way of a symmetric mode field distribution. Ina preferred embodiment the slab photonic crystal receives an incidentelectromagnetic beam having a particular state of parity and/or aparticular mode field distribution and preserves that state of parityand/or mode field distribution as it guides the electromagnetic beamduring transmission to produce an output electromagnetic beam havingcharacteristics that substantially match the characteristics of theincident beam.

The disclosure and discussion set forth herein is illustrative and notintended to limit the practice of the instant invention. While therehave been described what are believed to be the preferred embodiments ofthe instant invention, those skilled in the art will recognize thatother and further changes and modifications may be made thereto withoutdeparting from the spirit of the invention, and it is intended to claimall such changes and modifications that fall within the full scope ofthe invention. It is the following claims, including all equivalents, incombination with the foregoing disclosure and knowledge commonlyavailable to persons of skill in the art, which define the scope of theinstant invention.

1. A slab waveguide comprising a first cladding layer having a firstdielectric constant; a photonic crystal layer formed on said firstcladding layer, said photonic crystal layer including periodicallyarranged regions of a first dielectric material within a seconddielectric material, each of said periodically arranged regions of saidfirst dielectric material being asymmetric with respect to the mid-planeof said photonic crystal layer, said periodically arranged regionsinducing an asymmetry in the mode field distribution of an opticalsignal propagating within said photonic crystal layer; and a secondcladding layer formed on said photonic crystal layer, said secondcladding layer having a second dielectric constant, said seconddielectric constant differing from said first dielectric constant;wherein the difference between said first and second dielectricconstants causes said first and second cladding layers to offset saidasymmetry in said mode field distribution in said photonic crystal layerinduced by said periodically arranged regions.
 2. The waveguide of claim1, wherein said periodically arranged dielectric regions have a circularcross-section.
 3. The waveguide of claim 1, wherein said periodicallyarranged regions have a polygonal cross-section.
 4. The waveguide ofclaim 1, wherein said periodically arranged regions are tapered.
 5. Thewaveguide of claim 1, wherein the cross-sectional area of saidperiodically arranged dielectric regions varies between said firstcladding layer and said second cladding layer.
 6. The waveguide of claim1, wherein the cross-sectional shape of said periodically arrangeddielectric regions varies between said first cladding layer and saidsecond cladding layer.
 7. The waveguide of claim 1, wherein thedielectric constant of said first dielectric material is greater thanthe dielectric constant of said second dielectric material.
 8. Thewaveguide of claim 1, wherein the dielectric constant of said secondcladding layer is the same as the dielectric constant of said seconddielectric material.
 9. The waveguide of claim 1, wherein saidperiodically arranged dielectric regions extend continuously from saidfirst cladding layer to said second cladding layer.
 10. The waveguide ofclaim 1, wherein the mode field distribution supported by said waveguideis symmetric.
 11. The wavtguide of claim 10, wherein the plane ofsymmetry of said mode field distribution coincides with the mid-plane ofsaid photonic crystal layer.
 12. The waveguide of claim 10, wherein saidsymmetric mode field has even parity.
 13. The waveguide of claim 10,wherein said mode field distribution is preferentially localized in saidperiodically arranged dielectric regions.
 14. An electromagneticradiation transmission system comprising a source of electromagneticradiation, said source producing an input electromagnetic beam; and thewaveguide of claim 1, said waveguide receiving said inputelectromagnetic beam, said electromagnetic beam forming a guided mode insaid wavegoide, said guided mode having a mode field distribution insaid waveguide, said waveguide transmitting said guided mode to providean output beam of electromagnetic radiation.
 15. The transmission systemof claim 14, wherein said mode field distribution of said output beam issymmetric.
 16. The transmission system of claim 14, wherein said guidedmode has even parity.
 17. The transmission system of claim 14, whereinsaid output beam has the same parity as said input beam.
 18. Thetransmission system of claim 15, wherein said mode field distribution ofsaid input beam is symmetric.